The present invention relates to wireless communication networks, and particularly relates to controlling multiplexed signal transmission parameters responsive to improved received signal quality determination.
It is not uncommon for transmitters in wireless communication networks to perform transmit link adaptation responsive to received signal quality feedback from their targeted receivers. For example, any one or more of the transmit power, coding rate, modulation format, and the like, may be varied for a given receiver as function of the received signal quality reported by that receiver.
The use of received signal quality as a link adaptation control input is common for rate-controlled channels, wherein the network transmitter varies the data rate of a transmitted communication signal, rather than the transmit power of the signal, responsive to reported changes in received signal quality at the targeted receiver(s). The Wideband Code Division Multiple Access (W-CDMA) standards define a rate-controlled signal known as the High Speed Downlink Shared Channel (HS-DSCH). The HS-DSCH is a shared channel that provides high-rate packet data services to a potentially large number of receivers (users) that share the channel in time-multiplexed and/or code-multiplexed fashion.
The HS-DSCH is time-slotted and individual time slots are dedicated to individual users according to a defined scheduling algorithm. Users are scheduled according to service needs and other criteria, but the rate selected for serving a given user on that user's scheduled time slots generally is selected as a function of the received signal quality reported by the user. More particularly, the user reports a channel quality indicator, or some other representation of received signal quality, and the transmitter uses the reported value to determine the highest rate that can be supported by the user at acceptable error rates. Keeping the transmission error rate relatively low improves the overall system efficiency, because excessive data re-transmissions are avoided. Indeed, the effective throughput of the HS-DSCH can fall off quite rapidly if the transmitter selects data rates that are higher than are appropriate for the actual reception conditions at the targeted receivers.
Thus, if one or more of the targeted receivers “over reports” its received signal quality, the transmitter likely will make transmit link adaptations, e.g., the selection of transmit modulation formats and coding rates, that are inappropriate for the actual received signal quality at the misreporting receiver. The adaptation of the HS-DSCH to spatial multiplexing transmitters represents one circumstance where the targeted receivers are likely to report higher-than-actual received signal qualities. Similar over-reporting scenarios arise in code multiplexing transmissions (e.g., “multi-coding”), and in combinations of code multiplexing and spatial multiplexing.
The likelihood of over-reporting signal quality arises as a consequence of multiplexed transmission, such in spatial multiplexing transmissions, wherein code symbols belonging to the same codeword are transmitted as different signal streams and experience different fading and/or interference between the transmitter and the targeted receiver. Consequently, the received signal quality, e.g., the symbol signal-to-noise-plus-interference ratio (SINR), varies across the codeword at the input to the receiver's decoder. As an example, with four signal streams comprising a received multiplexed signal, the receiver experiences four distinct (stream-specific) received signal qualities. These variations in stream-specific received signal qualities give rise to signal quality losses at the receiver. Thus, simply reporting an average of the stream-specific received signal qualities does not provide the transmitter with an accurate “picture” of the true received signal quality at the receiver.